It looks to me like you only have a simple dial up service. That would be 56 kbps not 512 kbps. So the speeds you're getting for dial up are normal. But then again you said you were getting 15 KB/s which is faster than normal dial up speed. I wonder if you have broadband but are using a non-broadband modem?
Only adsl, dsl, (broadband), or cable or "some" satellite services will give you a faster service & overall speeds.

BTW to find the OSX firewall, open the system preferences. ie: Click on the apple icon on toolbar in far left, then choose system prefs & then open sharing & then click on the firewalls tab. More about it here Firewall configuration instructions (click on link) or see here http://www.gnutellaforums.com/showth...threadid=52529